Previously, epoxy resin compositions have been used as encapsulating materials for photosemiconductors such as light receiving elements and light emitting elements from the view point that the encapsulating materials should be excellent in transparency, moisture resistance and heat resistance.
However, conventional epoxy resin compositions are excellent in transparency, but have the problem that when the epoxy resin compositions are formed, for example, transfer molded, in forming molds in which photosemiconductor elements are arranged to encapsulate the photosemiconductor elements with the resins, thereby producing photosemiconductor devices, the mold releasing property is extremely deteriorated in taking the photosemiconductor devices out of the forming molds. Accordingly, various problems arise such as deformation of the photosemiconductor devices due to stress on mold releasing, generation of cracks in packages, separation of the photosemiconductor elements from the encapsulating resins, and separation of gold wires.
In general, when epoxy resin compositions are transfer molded, releasing agents are added to the epoxy resin compositions for improving the mold releasing property of moldings. When the photosemiconductor elements are encapsulated with the epoxy resin compositions by transfer molding as described above, addition of releasing agents for improving the mold releasing property without deteriorating the transparency of the epoxy resin compositions has been proposed (JP-A-6-157817 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), WO096/15191).
It is considered that the mold releasing property appears due to the presence of the releasing agent on the interface between the epoxy resin composition and mold in forming. When the photosemiconductor element is encapsulated with the epoxy resin composition by forming, for example, transfer molding, it is considered that the releasing agent is also present on the interface between the epoxy resin composition and photosemiconductor element. In that case, the releasing agent may cause separation of the photosemiconductor element from the epoxy resin composition at the interface between them in a formed photosemiconductor device, sometimes resulting in impairment of the function of the photosemiconductor device.